lookingGlass: Only update window list when visible

Updating the window list in the Looking Glass is a costly
operation: it destroys a whole lot of actors, and recreates
them. This triggers CSS changes, repaints, and allocations.

It is specially bad when paired with Wayland's big number
of window creations and deletions when showing Builder's
and Epiphany's popup window.

Only update the window list in the Looking Glass when it is
visible.

Related: https://gitlab.gnome.org/GNOME/mutter/issues/556

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/719

js/ui/lookingGlass.js

@@ -304,6 +304,9 @@ var WindowList = class WindowList {
     }
 
     _updateWindowList() {
+        if (!this._lookingGlass.isOpen)
+            return;
+
         this.actor.destroy_all_children();
         let windows = global.get_window_actors();
         let tracker = Shell.WindowTracker.get_default();
@@ -335,6 +338,10 @@ var WindowList = class WindowList {
             }
         }
     }
+
+    update() {
+        this._updateWindowList();
+    }
 };
 Signals.addSignalMethods(WindowList.prototype);
 
@@ -1099,6 +1106,8 @@ var LookingGlass = class LookingGlass {
             duration,
             mode: Clutter.AnimationMode.EASE_OUT_QUAD
         });
+
+        this._windowList.update();
     }
 
     close() {
@@ -1124,5 +1133,9 @@ var LookingGlass = class LookingGlass {
             onComplete: () => this.actor.hide()
         });
     }
+
+    get isOpen() {
+        return this._open;
+    }
 };
 Signals.addSignalMethods(LookingGlass.prototype);